Ed,
Ask and you shall receive.
I just finished the drawings for a receiver & cover that have a 0.300" wide opening. I'll be ordering some to try.
The covers slide in and held in place with a tab at the front, and then the screw holds it all under the car. The vertical blocks prevent deformation of the plastic during tightening and allow for the spring to have a flat surface to push off besides the screw.
The coupler should self-center as the block should cause any angle to work itself out at the rear of the shank. The coupler will slide into the front of the box and up and over the box (I've clearance it accordingly), and then when you slide the cover in place it will fill in that clearance at the front of the opening.
Stay tuned.
PS - Shapeways has 25% & free shipping today only.
Thanks,
Mario